Since the Hall effect thruster is known, its structure and its operating principle are briefly summarized below.
FIG. 2 is a view in perspective and partial section showing a Hall effect thruster 1. Around a central core 10 extending along a longitudinal axis A, there is situated a central magnetic coil 12. An inner wall 20 of annular shape surrounds the central magnetic coil 12 and the central core 10. The inner wall 20 is surrounded by an outer wall 40 of annular shape, such that between them these two walls define an annular channel extending along the axis A and referred to as the discharge channel 50.
In the description below, the term “inner” designates a portion that is closer to the axis A, and the term “outer” designates a portion that is further from the axis A.
The upstream end of the discharge channel 50 is closed by an injector system 30 that injects atoms into the discharge channel 50, and that also constitutes an anode. The downstream end 52 of the discharge channel 50 is open.
A plurality of peripheral magnetic coils 14 are situated around the outer wall 40. The central magnetic coil 12 and the peripheral magnetic coil 14 serve to generate a radial magnetic field B of intensity that is at a maximum towards the downstream end 52 of the discharge channel 50.
A hollow cathode 100 is situated outside the outer wall 40, and a potential difference is established between the cathode 100 and the anode (injector system 30). The hollow cathode 100 is positioned in such a manner as to eject electrons in the vicinity of the downstream end 52 of the discharge channel 50.
Inside the discharge channel 50, these electrons head towards the injector system 30 under the influence of the electric field generated by the potential difference between the cathode 100 and the anode, however some of them are trapped by the magnetic field B close to the downstream opening 52 of the discharge channel 50.
The electrons are thus caused to describe circumferential trajectories in the discharge channel 50 at its downstream opening 52. By impact, these electrons then ionize atoms of inert gas (generally xenon Xe) flowing from upstream to downstream in the discharge channel 50, thereby creating ions. These electrons also create an axial electric field E that accelerates the ions away from the anode (injector system 30 at the bottom of the channel 80) towards the downstream opening 52, such that the ions are ejected at high speed from the discharge channel 50 through its downstream end 52, thereby generating the thrust of the thruster.
When starting the Hall effect thruster, and after a repeated number of such starts, the operation of the Hall effect thruster is observed to become unstable, i.e. ions are ejected from the discharge channel in a manner that is not stable over time. This instability generates magnetic emissions that lead to insufficient performance from the Hall effect thruster.
This instability can be minimized by reducing the voltage between the cathode and the anode while starting. However that solution reduces the overall performance of the Hall effect thruster.
It is also possible to correct the instability by modifying the magnetic field B. However that correction requires an additional electronic device to be installed and used, thus necessarily consuming energy and thus making the Hall effect thruster more expensive to fabricate and presenting a lifetime that is shorter.